The present invention relates to providing cover, layer envelope, or jacket upon a steel pipe under utilization of, e.g., a thermoplastic material, in particular polyethylene; and more particularly, the invention relates to helically wind a ribbon, such as a thermoplastic ribbon, upon a steel pipe, the pipe's surface having been covered with a suitable adhesive.
The German printed Pat. No. 17,71,764 describes a device in which a freshly extruded polyethylene ribbon is wrapped around a steel pipe of large diameter. The pipe rotates and advances axially by means of a suitable roller track, resulting in a helical wrapping of the ribbon. A roller urges the ribbon onto the pipe. The adhesive is applied by freshly extruding a ribbon of an adhesive, and wrapping it also onto the pipe to be located between the pipe's surface and the polyethylene ribbon. The roller used to urge and force the plastic ribbon onto the pipe serves for smoothing any unevenness such as may result from an overlap. Also, overlapping edges of the freshly extruded ribbon will be welded together by operation of this pressure roll.
In order to optimize equipment use, sequential pipes should pass through the wrapping and jacketing station in end-to-end relation. However, pipes being curved to some extent, or having an oval cross section, will not be wrapped properly at their two ends because the pressure roll may not work properly on both of the two pipes, which are supposedly abutting but are locally misaligned with level differences of up to 50 mm. Thus, pipes exhibiting any deviation from a true, circular, cylindrical surface will usually pass instead through the wrapping station at a certain distance from each other, such as 250 mm to 300 mm. That will not reduce tumbling of the pipes relative to each other, but does reduce distortion of the ribbon as no uneven joint passes. This approach, however, is disadvantaged by the incurrence of considerable waste because the gap space between the pipes is "wrapped."
Aside from the foregoing, one has to consider the need for separating the pipes from each other which requires cutting the wrapped ribbon. This is quite dangerous because the ribbon is approximately 200.degree. C. hot. Thus, cooling (i,e., a delay) is required before the waste can be handled any further. The waste can, of course, be recycled to some extent; i.e., it may be ground and granulated to be used again in the extruder. Nevertheless, such supplemental processing of the waste is required to save at least some of it.